Bell-Magendie law

The anterior spinal nerve roots contain only motor fibres and posterior roots only sensory fibres.

Charles Bell's work of 1811 Contains the first reference to experimental work on the motor functions the ventral spinal nerve without, however, establishing the sensory functions of the dorsal roots.

In 1822 Magendie definitively discovered that the anterior root is motor and that the dorsal root is sensory. Magendie announced that "section of the dorsal root abolishes sensation, section of ventral roots abolishes motor activity, and section of both roots abolishes both sensation and motor activity". This discovery has been called "the most momentous single discovery in physiology after Harvey". In the same volume of Journal de physiologie expérimentale et de pathologie, Magendie gave experimental proof of the Bell-Magendie law.

Magendie proved Bell’s law by severing the anterior and posterior roots of spinal nerves in a litter of puppies. Stimulation of the posterior roots caused pain.

«In sum, Charles Bell had had, before me, but unknown to me, the idea of separately cutting the spinal roots; he likewise discovered that the anterior influences muscular contractility more than the posterior does. This is a question of priority in which I have, from the beginning, honored him. Now, as for having established that these roots have distinct properties, distinct functions, that the anterior ones control movement, and the posterior ones sensation, this discovery belongs to me»
Magendie in Comptes rendus hebdomadaires des séances
de l’Académie des sciences, 1847, 24: 320).

At first Müller experimented on rabbits, but the work was difficult and yielded ambiguous results. Consequently he continued his investigation on frogs, the use of which in the laboratory had almost entirely ceased. In the frog the spinal cord was far easier to remove, the relationships between the nerve roots were much more apparent, and the results unambiguous and always reproducible. Cutting through the posterior roots leading to a hind leg, he found that the limb became insensible but was not paralysed. When he cut the anterior roots, however, he observed that the limb was paralysed but not rendered insensible. The simplicity, conclusiveness, and memorableness of this experiment - which has been repeated countless times in physiology courses – made a marked impression on Müller’s contemporaries.

A historical study of the discovery of the Magendie-Bell doctrines is found in [Pflüger’s] Archiv für die gesamte Physiologie des Menschen und der Tiere, Berlin, 1901, 84: 276-303.

• C. Bell:
  An Idea of a New Anatomy of the Brain; submitted for the observations of his friends.
  A privately printed pamphlet. London, Strahan & Preston, 1811.
• F. Magendie:
  Expériences sur les fonctions des racines des nerfs rachidiens.
  Journal de physiologie expérimentale et de pathologie, 1822; 2: 276-279.
• F. Magendie:
  Expériences sur les fonctions des racines des nerfs qui naissent de la moëlle épinière.
  Journal de physiologie expérimentale et de pathologie, 1822, 2: 366-371.
  Experimental proof of the Bell-Magendie law. However, Johannes Müller is commonly credited the first proof of this law.
• J. Müller:
  Bestätigung des Bell'schen Lehrsatzes, dass die doppelten Wurzeln der Rückenmarksnerven verschiedene Functionen, durch neue und entscheidende Experimente.
  [Froriep's]Notizen aus dem Gebiete der Natur- und Heilkunde, Weimar, 1831, 30, 113-117, 129-134.
• Paul Frederick Cranefield (1925–):
  The way in and the way out. François Magendie, Charles Bell and the roots of the spinal nerves.
  Mt. Kisco, N.Y., Futura Publishing, 1974. Cranefield proves that Magendie discovered the Bell-Magendie law.